Plexin-A3 and plexin-A4 restrict the migration of sympathetic neurons but not their neural crest precursors

Motor innervation directs the correct development of the mouse sympathetic nervous system

The sympathetic nervous system controls bodily functions including vascular tone, cardiac rhythm, and the "fight-or-flight response". Sympathetic chain ganglia develop in parallel with preganglionic motor nerves extending from the neural tube, raising the question of whether axon targeting contributes to sympathetic chain formation. Using nerve-selective genetic ablations and lineage tracing in mouse, we reveal that motor nerve-associated Schwann cell precursors (SCPs) contribute sympathetic neurons and satellite glia after the initial seeding of sympathetic ganglia by neural crest. Motor nerve ablation causes mispositioning of SCP-derived sympathoblasts as well as sympathetic chain hypoplasia and fragmentation. Sympathetic neurons in motor-ablated embryos project precociously and abnormally towards dorsal root ganglia, eventually resulting in fusion of sympathetic and sensory ganglia. Cell interaction analysis identifies semaphorins as potential motor nerve-derived signaling molecules regulating sympathoblast positioning and outgrowth. Overall, central innervation functions both as infrastructure and regulatory niche to ensure the integrity of peripheral ganglia morphogenesis.

Pathophysiological functions of semaphorins in the sympathetic nervous system

Upon exposure to external stressors, the body senses them and activates the sympathetic nervous system (SNS) to maintain the homeostasis, which is known as the "fight-or-flight" response. Recent studies have revealed that the SNS also plays pivotal roles in regulating immune responses, such as hematopoiesis, leukocyte mobilization, and inflammation. Indeed, overactivation of the SNS causes many inflammatory diseases, including cardiovascular diseases, metabolic disorders, and autoimmune diseases. However, the molecular basis essential for SNS-mediated immune regulation is not completely understood. In this review, we focus on axon guidance cues, semaphorins, which play multifaceted roles in neural and immune systems. We summarize the functions of semaphorins in the crosstalk between the SNS and the immune system, exploring its pathophysiological roles.

PlexinA4-Semaphorin3A-mediated crosstalk between main cortical interneuron classes is required for superficial interneuron lamination

In the mammalian cerebral cortex, the developmental events governing allocation of different classes of inhibitory interneurons (INs) to distinct cortical layers are poorly understood. Here we report that the guidance receptor PlexinA4 (PLXNA4) is upregulated in serotonin receptor 3a-expressing (HTR3A⁺) cortical INs (hINs) as they invade the cortical plate, and that it regulates their laminar allocation to superficial cortical layers. We find that the PLXNA4 ligand Semaphorin3A (SEMA3A) acts as a chemorepulsive factor on hINs migrating into the nascent cortex and demonstrate that SEMA3A specifically controls their laminar positioning through PLXNA4. We identify deep-layer INs as a major source of SEMA3A in the developing cortex and demonstrate that targeted genetic deletion of Sema3a in these INs specifically affects laminar allocation of hINs. These data show that, in the neocortex, deep-layer INs control laminar allocation of hINs into superficial layers.

Modular and Distinct Plexin-A4/FARP2/Rac1 Signaling Controls Dendrite Morphogenesis

Diverse neuronal populations with distinct cellular morphologies coordinate the complex function of the nervous system. Establishment of distinct neuronal morphologies critically depends on signaling pathways that control axonal and dendritic development. The Sema3A-Nrp1/PlxnA4 signaling pathway promotes cortical neuron basal dendrite arborization but also repels axons. However, the downstream signaling components underlying these disparate functions of Sema3A signaling are unclear. Using the novel PlxnA4^KRK-AAA knock-in male and female mice, generated by CRISPR/cas9, we show here that the KRK motif in the PlxnA4 cytoplasmic domain is required for Sema3A-mediated cortical neuron dendritic elaboration but is dispensable for inhibitory axon guidance. The RhoGEF FARP2, which binds to the KRK motif, shows identical functional specificity as the KRK motif in the PlxnA4 receptor. We find that Sema3A activates the small GTPase Rac1, and that Rac1 activity is required for dendrite elaboration but not axon growth cone collapse. This work identifies a novel Sema3A-Nrp1/PlxnA4/FARP2/Rac1 signaling pathway that specifically controls dendritic morphogenesis but is dispensable for repulsive guidance events. Overall, our results demonstrate that the divergent signaling output from multifunctional receptor complexes critically depends on distinct signaling motifs, highlighting the modular nature of guidance cue receptors and its potential to regulate diverse cellular responses.SIGNIFICANCE STATEMENT The proper formation of axonal and dendritic morphologies is crucial for the precise wiring of the nervous system that ultimately leads to the generation of complex functions in an organism. The Semaphorin3A-Neuropilin1/Plexin-A4 signaling pathway has been shown to have multiple key roles in neurodevelopment, from axon repulsion to dendrite elaboration. This study demonstrates that three specific amino acids, the KRK motif within the Plexin-A4 receptor cytoplasmic domain, are required to coordinate the downstream signaling molecules to promote Sema3A-mediated cortical neuron dendritic elaboration, but not inhibitory axon guidance. Our results unravel a novel Semaphorin3A-Plexin-A4 downstream signaling pathway and shed light on how the disparate functions of axon guidance and dendritic morphogenesis are accomplished by the same extracellular ligand in vivo.

Reconstruction of the Global Neural Crest Gene Regulatory Network In Vivo

Precise control of developmental processes is encoded in the genome in the form of gene regulatory networks (GRNs). Such multi-factorial systems are difficult to decode in vertebrates owing to their complex gene hierarchies and dynamic molecular interactions. Here we present a genome-wide in vivo reconstruction of the GRN underlying development of the multipotent neural crest (NC) embryonic cell population. By coupling NC-specific epigenomic and transcriptional profiling at population and single-cell levels with genome/epigenome engineering in vivo, we identify multiple regulatory layers governing NC ontogeny, including NC-specific enhancers and super-enhancers, novel trans-factors, and cis-signatures allowing reverse engineering of the NC-GRN at unprecedented resolution. Furthermore, identification and dissection of divergent upstream combinatorial regulatory codes has afforded new insights into opposing gene circuits that define canonical and neural NC fates early during NC ontogeny. Our integrated approach, allowing dissection of cell-type-specific regulatory circuits in vivo, has broad implications for GRN discovery and investigation.

Venous endothelin modulates responsiveness of cardiac sympathetic axons to arterial semaphorin

Developing neurons of the peripheral nervous system reach their targets via cues that support directional growth, a process known as axon guidance. In investigating how sympathetic axons reach the heart in mice, we discovered that a combination of guidance cues are employed in sequence to refine axon outgrowth, a process we term second-order guidance. Specifically, endothelin-1 induces sympathetic neurons expressing the receptor Ednra to project to the vena cavae leading to the heart. Endothelin signaling in turn induces expression of the repulsive receptor Plexin-A4, via induction of the transcription factor MEF2C. In the absence of endothelin or plexin signaling, sympathetic neurons misproject to incorrect competing vascular trajectories (the dorsal aorta and intercostal arteries). The same anatomical and physiological consequences occur in Ednra^+/-; Plxna4^+/- double heterozygotes, genetically confirming functional interaction. Second-order axon guidance therefore multiplexes a smaller number of guidance cues in sequential fashion, allowing precise refinement of axon trajectories.

Toward the Existence of a Sympathetic Neuroplasticity Adaptive Mechanism Influencing the Immune Response. A Hypothetical View-Part I

The nervous system exerts a profound influence on the function of the immune system (IS), mainly through the sympathetic arm of the autonomic nervous system. In fact, the sympathetic nervous system richly innervates secondary lymphoid organs (SLOs) such as the spleen and lymph nodes. For decades, different research groups working in the field have consistently reported changes in the sympathetic innervation of the SLOs during the activation of the IS, which are characterized by a decreased noradrenergic activity and retraction of these fibers. Most of these groups interpreted these changes as a pathological phenomenon, referred to as "damage" or "injury" of the noradrenergic fibers. Some of them postulated that this "injury" was probably due to toxic effects of released endogenous mediators. Others, working on animal models of chronic stimulation of the IS, linked it to the very chronic nature of processes. Unlike these views, this first part of the present work reviews evidence which supports the hypothesis of a specific adaptive mechanism of neural plasticity from sympathetic fibers innervating SLOs, encompassing structural and functional changes of noradrenergic nerves. This plasticity mechanism would involve segmental retraction and degeneration of these fibers during the activation of the IS with subsequent regeneration once the steady state is recovered. The candidate molecules likely to mediate this phenomenon are also here introduced. The second part will extend this view as to the potential changes in sympathetic innervation likely to occur in inflamed non-lymphoid peripheral tissues and its possible immunological implications.

PLXNA3 Variant rs5945430 is Associated with Severe Clinical Course in Male Multiple Sclerosis Patients

Multiple sclerosis (MS) exhibits sex bias in disease clinical course as male MS patients develop severe, progressive clinical course with accumulating disability. So far, no factors have been found associating with this sex bias in MS severity. We set out to determine the genetic factor contributing to MS male-specific progressive disease. This is an MS cross-sectional study involving 213 Kuwaiti MS patients recruited at Dasman Diabetes Institute. Exome sequencing was performed on 18 females and 8 male MS patients' genomic DNA. rs5945430 genotyping was performed using Taqman genotyping assay. Estradiol levels were determined by enzyme-linked immunosorbent assay. Exome analysis revealed a missense variant (rs5945430) in Plexin A3 (PLXNA3) gene (Xq28) associated with male-specific MS severity. Genotyping of 187 MS patients for rs5945430 confirmed the association of rs5945430G with increased disease severity in MS males (p = 0.013; OR 3.8; 95% CI 1.24-11.7) and disability (p = 0.024). Estradiol levels shown to effect PLXNA3 expression were lower in MS males compared to MS females, and they were lower than control rs5945430G males (p = 0.057), whereas MS females had similar estradiol levels to healthy females reducing the level of expressed PLXNA3 GG in MS females. PLXNA3 rs5945430G is associated with increased disease severity in MS male patients. Estradiol is a possible protective factor against the expression of rs5945430G in MS females.

Transcriptome profiling reveals expression signatures of cranial neural crest cells arising from different axial levels

Background

Cranial neural crest cells (NCCs) are a unique embryonic cell type which give rise to a diverse array of derivatives extending from neurons and glia through to bone and cartilage. Depending on their point of origin along the antero-posterior axis cranial NCCs are rapidly sorted into distinct migratory streams that give rise to axial specific structures. These migratory streams mirror the underlying segmentation of the brain with NCCs exiting the diencephalon and midbrain following distinct paths compared to those exiting the hindbrain rhombomeres (r). The genetic landscape of cranial NCCs arising at different axial levels remains unknown.

Results

Here we have used RNA sequencing to uncover the transcriptional profiles of mouse cranial NCCs arising at different axial levels. Whole transcriptome analysis identified over 120 transcripts differentially expressed between NCCs arising anterior to r3 (referred to as r1-r2 migratory stream for simplicity) and the r4 migratory stream. Eight of the genes differentially expressed between these populations were validated by RT-PCR with 2 being further validated by in situ hybridisation. We also explored the expression of the Neuropilins (Nrp1 and Nrp2) and their co-receptors and show that the A-type Plexins are differentially expressed in different cranial NCC streams.

Conclusions

Our analyses identify a large number of genes differentially regulated between cranial NCCs arising at different axial levels. This data provides a comprehensive description of the genetic landscape driving diversity of distinct cranial NCC streams and provides novel insight into the regulatory networks controlling the formation of specific skeletal elements and the mechanisms promoting migration along different paths.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0147-z) contains supplementary material, which is available to authorized users.

Reduced Sympathetic Innervation in Endometriosis is Associated to Semaphorin 3C and 3F Expression

Endometriosis is a chronic inflammatory disease and one of the most common causes of pelvic pain. The mechanisms underlying pain emergence or chronic inflammation during endometriosis remain unknown. Several chronic inflammatory diseases including endometriosis show reduced amounts of noradrenergic nerve fibers. The source of the affected innervation is still unclear. Semaphorins represent potential elicitors, due to their known role as axonal guidance cues, and are suggested as nerve repellent factors in different chronic inflammatory diseases. Therefore, semaphorins might influence the progress of neuroinflammatory mechanisms during endometriosis. Here, we analyzed the noradrenergic innervation and the expression of the specific semaphorins and receptors possibly involved in the neuroimmunomodulation in endometriosis. Our studies revealed an affected innervation and a significant increase of semaphorins and their receptors in peritoneal endometriotic tissue. Thereby, the expression of the receptors was identified on the membrane of noradrenergic nerve fibers and vessels. Macrophages and activated fibroblasts were found in higher density levels and additionally express semaphorins in peritoneal endometriotic tissue. Inflammation leads to an increased release of immune cells, which secrete a variety of inflammatory factors capable of affecting innervation. Therefore, our data suggests that the chronic inflammatory condition in endometriosis might contribute to the increase of semaphorins, which could possibly affect the innervation in peritoneal endometriosis.

Potential role of estrogen in maintaining the imbalanced sympathetic and sensory innervation in endometriosis

Endometriosis, one of the most common benign gynecological diseases, affects millions of women of childbearing age. Endometriosis-associated pain is a major cause of disability and compromised quality of life in women. Neuropathic mechanisms are believed to play an important role. An imbalanced sympathetic and sensory innervation (reduced sympathetic innervation, with unchanged or increased sensory innervation in endometriotic lesions) has been demonstrated in endometriosis in recent studies. And it is believed to contribute to the pathogenesis of endometriosis-associated pain. It is primarily considered to be a natural adaptive program to endometriosis-associated inflammation. However, it is important to further clarify whether other potential modulating factors are involved in this dysregulation. It is generally accepted that endometriosis is an estrogen dependent disease. Higher estrogen biosynthesis and lower estrogen inactivation in endometriosis can lead to an excess of local estrogen in endometriotic lesions. In addition to its proliferative and anti-inflammatory actions, local estrogen in endometriosis also exerts potential neuromodulatory effects on the innervation in endometriosis. The aim of this review is to highlight the role of estrogen in mediating this imbalanced sympathetic and sensory innervation in endometriosis, through direct and indirect mechanisms on sympathetic and sensory nerves. Theoretical elaboration of the underlying mechanisms provides new insights in supporting the therapeutic role of estrogen in endometriosis-associated pain.

Identification of molecular pathways facilitating glioma cell invasion in situ

Gliomas are mostly incurable secondary to their diffuse infiltrative nature. Thus, specific therapeutic targeting of invasive glioma cells is an attractive concept. As cells exit the tumor mass and infiltrate brain parenchyma, they closely interact with a changing micro-environmental landscape that sustains tumor cell invasion.

In this study, we used a unique microarray profiling approach on a human glioma stem cell (GSC) xenograft model to explore gene expression changes in situ in Invading Glioma Cells (IGCs) compared to tumor core, as well as changes in host cells residing within the infiltrated microenvironment relative to the unaffected cortex. IGCs were found to have reduced expression of genes within the extracellular matrix compartment, and genes involved in cell adhesion, cell polarity and epithelial to mesenchymal transition (EMT) processes. The infiltrated microenvironment showed activation of wound repair and tissue remodeling networks. We confirmed by protein analysis the downregulation of EMT and polarity related genes such as CD44 and PARD3 in IGCs, and EFNB3, a tissue-remodeling agent enriched at the infiltrated microenvironment. OLIG2, a proliferation regulator and glioma progenitor cell marker upregulated in IGCs was found to function in enhancing migration and stemness of GSCs. Overall, our results unveiled a more comprehensive picture of the complex and dynamic cell autonomous and tumor-host interactive pathways of glioma invasion than has been previously demonstrated. This suggests targeting of multiple pathways at the junction of invading tumor and microenvironment as a viable option for glioma therapy.

Plexin a4 expression in adult rat cranial nerves

PlexinsA1-A4 participate in class 3 semaphorin signaling as co-receptors to neuropilin 1 and 2. PlexinA4 is the latest member of the PlexinA subfamily to be identified. In previous studies, we described the expression of PlexinA4 in the brain and spinal cord of the adult rat. Here, antibodies to PlexinA4 were used to reveal immunolabeling in most of the cranial nerve surveyed. Labeling was found in the olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, and hypoglossal nerves. This is the first detailed description of the cellular and subcellular distribution of PlexinA4 in the adult cranial nerves. The findings will set the basis for future studies on the potential role of PlexinA4 in regeneration and repair of the adult central and peripheral nervous system.

PLXNA4 is associated with Alzheimer disease and modulates tau phosphorylation

OBJECTIVE

Much of the genetic basis for Alzheimer disease (AD) is unexplained. We sought to identify novel AD loci using a unique family-based approach that can detect robust associations with infrequent variants (minor allele frequency < 0.10).

METHODS

We conducted a genome-wide association study in the Framingham Heart Study (discovery) and NIA-LOAD (National Institute on Aging-Late-Onset Alzheimer Disease) Study (replication) family-based cohorts using an approach that accounts for family structure and calculates a risk score for AD as the outcome. Links between the most promising gene candidate and AD pathogenesis were explored in silico as well as experimentally in cell-based models and in human brain.

RESULTS

Genome-wide significant association was identified with a PLXNA4 single nucleotide polymorphism (rs277470) located in a region encoding the semaphorin-3A (SEMA3A) binding domain (meta-analysis p value [meta-P] = 4.1 × 10(-8) ). A test for association with the entire region was also significant (meta-P = 3.2 × 10(-4) ). Transfection of SH-SY5Y cells or primary rat neurons with full-length PLXNA4 (TS1) increased tau phosphorylation with stimulated by SEMA3A. The opposite effect was observed when cells were transfected with shorter isoforms (TS2 and TS3). However, transfection of any isoform into HEK293 cells stably expressing amyloid β (Aβ) precursor protein (APP) did not result in differential effects on APP processing or Aβ production. Late stage AD cases (n = 9) compared to controls (n = 5) had 1.9-fold increased expression of TS1 in cortical brain tissue (p = 1.6 × 10(-4) ). Expression of TS1 was significantly correlated with the Clinical Dementia Rating score (ρ = 0.75, p = 2.2 × 10(-4) ), plaque density (ρ = 0.56, p = 0.01), and Braak stage (ρ = 0.54, p = 0.02).

INTERPRETATION

Our results indicate that PLXNA4 has a role in AD pathogenesis through isoform-specific effects on tau phosphorylation.

Signaling molecules and transcription factors involved in the development of the sympathetic nervous system, with special emphasis on the superior cervical ganglion

The cells that constitute the sympathetic nervous system originate from the neural crest. This review addresses the current understanding of sympathetic ganglion development viewed from molecular and morphological perspectives. Development of the sympathetic nervous system is categorized into three main steps, as follows: (1) differentiation and migration of cells in the neural crest lineage for formation of the primary sympathetic chain, (2) differentiation of sympathetic progenitors, and (3) growth and survival of sympathetic ganglia. The signaling molecules and transcription factors involved in each of these developmental stages are elaborated mostly on the basis of the results of targeted mutation of respective genes. Analyses in mutant mice revealed differences between the superior cervical ganglion (SCG) and the other posterior sympathetic ganglia. This review provides a summary of the similarities and differences in the development of the SCG and other posterior sympathetic ganglia. Relevant to the development of sympathetic ganglia is the demonstration that neuroendocrine cells, such as adrenal chromaffin cells and carotid body glomus cells, share a common origin with the sympathetic ganglia. Neural crest cells at the trunk level give rise to common sympathoadrenal progenitors of sympathetic neurons and chromaffin cells, while progenitors segregated from the SCG give rise to glomus cells. After separation from the sympathetic primordium, the progenitors of both chromaffin cells and glomus cells colonize the anlage of the adrenal gland and carotid body, respectively. This review highlights the biological properties of chromaffin cells and glomus cells, because, although both cell types are derivatives of sympathetic primordium, they are distinct in many respects.

Lead exposure disrupts global DNA methylation in human embryonic stem cells and alters their neuronal differentiation

Exposure to lead (Pb) during childhood can result in learning disabilities and behavioral problems. Although described in animal models, whether Pb exposure also alters neuronal differentiation in the developing brains of exposed children is unknown. Here, we investigated the effects of physiologically relevant concentrations of Pb (from 0.4 to 1.9μM) on the capacity of human embryonic stem cells (hESCs) to progress to a neuronal fate. We found that neither acute nor chronic exposure to Pb prevented hESCs from generating neural progenitor cells (NPCs). NPCs derived from hESCs chronically exposed to 1.9μM Pb throughout the neural differentiation process generated 2.5 times more TUJ1-positive neurons than those derived from control hESCs. Pb exposure of hESCs during the stage of neural rosette formation resulted in a significant decrease in the expression levels of the neural marker genes PAX6 and MSI1. Furthermore, the resulting NPCs differentiated into neurons with shorter neurites and less branching than control neurons, as assessed by Sholl analysis. DNA methylation studies of control, acutely treated hESCs and NPCs derived from chronically exposed hESCs using the Illumina HumanMethylation450 BeadChip demonstrated that Pb exposure induced changes in the methylation status of genes involved in neurogenetic signaling pathways. In summary, our study shows that exposure to Pb subtly alters the neuronal differentiation of exposed hESCs and that these changes could be partly mediated by modifications in the DNA methylation status of genes crucial to brain development.

Rare variants in PLXNA4 and Parkinson's disease

Approximately 20% of individuals with Parkinson's disease (PD) report a positive family history. Yet, a large portion of causal and disease-modifying variants is still unknown. We used exome sequencing in two affected individuals from a family with late-onset familial PD followed by frequency assessment in 975 PD cases and 1014 ethnically-matched controls and linkage analysis to identify potentially causal variants. Based on the predicted penetrance and the frequencies, a variant in PLXNA4 proved to be the best candidate and PLXNA4 was screened for additional variants in 862 PD cases and 940 controls, revealing an excess of rare non-synonymous coding variants in PLXNA4 in individuals with PD. Although we cannot conclude that the variant in PLXNA4 is indeed the causative variant, these findings are interesting in the light of a surfacing role of axonal guidance mechanisms in neurodegenerative disorders but, at the same time, highlight the difficulties encountered in the study of rare variants identified by next-generation sequencing in diseases with autosomal dominant or complex patterns of inheritance.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

Neuropilin signalling in vessels, neurons and tumours

The neuropilins NRP1 and NRP2 are transmembrane proteins that regulate many different aspects of vascular and neural development. Even though they were originally identified as adhesion molecules, they are most commonly studied as co-receptors for secreted signalling molecules of the class 3 semaphorin (SEMA) and vascular endothelial growth factor (VEGF) families. During nervous system development, both classes of ligands control soma migration, axon patterning and synaptogenesis in the central nervous system, and they additionally help to guide the neural crest cell precursors of neurons and glia in the peripheral nervous system. Both classes of neuropilin ligands also control endothelial cell behaviour, with NRP1 acting as a VEGF-A isoform receptor in blood vascular endothelium and as a semaphorin receptor in lymphatic valve endothelium, and NRP2 promoting lymphatic vessel growth induced by VEGF-C. Here we provide an overview of neuropilin function in neurons and neural crest cells, discuss current knowledge of neuropilin signalling in the vasculature and conclude with a summary of neuropilin roles in cancer.

Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin transmembrane receptors and their semaphorin ligands, as well as their co-receptors (Neuropilin, Integrin, VEGFR2, ErbB2, and Met kinase) are emerging as key regulatory proteins in a wide variety of developmental, regenerative, but also pathological processes. The diverse arenas of plexin function are surveyed, including roles in the nervous, cardiovascular, bone and skeletal, and immune systems. Such different settings require considerable specificity among the plexin and semaphorin family members which in turn are accompanied by a variety of cell signaling networks. Underlying the latter are the mechanistic details of the interactions and catalytic events at the molecular level. Very recently, dramatic progress has been made in solving the structures of plexins and of their complexes with associated proteins. This molecular level information is now suggesting detailed mechanisms for the function of both the extracellular as well as the intracellular plexin regions. Specifically, several groups have solved structures for extracellular domains for plexin-A2, -B1, and -C1, many in complex with semaphorin ligands. On the intracellular side, the role of small Rho GTPases has been of particular interest. These directly associate with plexin and stimulate a GTPase activating (GAP) function in the plexin catalytic domain to downregulate Ras GTPases. Structures for the Rho GTPase binding domains have been presented for several plexins, some with Rnd1 bound. The entire intracellular domain structure of plexin-A1, -A3, and -B1 have also been solved alone and in complex with Rac1. However, key aspects of the interplay between GTPases and plexins remain far from clear. The structural information is helping the plexin field to focus on key questions at the protein structural, cellular, as well as organism level that collaboratoria of investigations are likely to answer.

Plexin A3 is involved in semaphorin 3F-mediated oligodendrocyte precursor cell migration

Class 3 semaphorins are expressed in the neurodevelopmental or damage repair phase of the central nervous system (CNS). They play an important role in guiding axon growth and directing cell migration, including the migration of oligodendrocyte precursor cells (OPCs). As co-receptors for semaphorin 3F(sema3F), the expression and role of neuropilin-2 (NRP2) and plexin A3 in OPC migration are unclear. Using RT-PCR, Western blot analysis, and immunofluorescence, we demonstrated that primary OPCs and immature oligodendrocytes from neonatal rats express NRP2 and plexin A3. After transfection with NRP2 siRNA and plexin A3 siRNA, the number of migrating OPCs attracted to sema3F remarkably decreased. These results suggest that plexin A3 is expressed in OPCs and immature oligodendrocytes and is involved in OPC migration.

NRP1 and NRP2 cooperate to regulate gangliogenesis, axon guidance and target innervation in the sympathetic nervous system

The sympathetic nervous system (SNS) arises from neural crest (NC) cells during embryonic development and innervates the internal organs of vertebrates to modulate their stress response. NRP1 and NRP2 are receptors for guidance cues of the class 3 semaphorin (SEMA) family and are expressed in partially overlapping patterns in sympathetic NC cells and their progeny. By comparing the phenotypes of mice lacking NRP1 or its ligand SEMA3A with mice lacking NRP1 in the sympathetic versus vascular endothelial cell lineages, we demonstrate that SEMA3A signalling through NRP1 has multiple cell-autonomous roles in SNS development. These roles include neuronal cell body positioning, neuronal aggregation and axon guidance, first during sympathetic chain assembly and then to regulate the innervation of the heart and aorta. Loss of NRP2 or its ligand SEMA3F impaired sympathetic gangliogenesis more mildly than loss of SEMA3A/NRP1 signalling, but caused ectopic neurite extension along the embryonic aorta. The analysis of compound mutants lacking SEMA3A and SEMA3F or NRP1 and NRP2 in the SNS demonstrated that both signalling pathways cooperate to organise the SNS. We further show that abnormal sympathetic development in mice lacking NRP1 in the sympathetic lineage has functional consequences, as it causes sinus bradycardia, similar to mice lacking SEMA3A.

Receptor complexes for each of the Class 3 Semaphorins

The Class 3 Semaphorins (Sema3s) are a sub-family of proteins whose known biological roles are varied and growing. The mechanism of action of the Sema3s requires binding to transmembrane receptors that comprise heteromeric complexes of Neuropilins, Plexins and cell adhesion molecules (CAMs). However, knowledge of the receptor components of the Sema3s remains incomplete, and there may be receptor components which are as yet undiscovered. The receptor complexes of the Sema3s share receptor components with each other, and it is the specific combination of these components within a heteromeric complex that is thought to give rise to selective binding and signalling for individual Sema3s. This crosstalk makes it experimentally difficult to define a single holoreceptor for each Sema3. Furthermore, the receptor composition for a given Sema3 may differ between cell types, and change as a function of developmental state or pathological situation. Nevertheless, there are at least some known differences in the constitutive structure of the receptors for the Sema3s. For example in neural cells, Sema3a and Sema3f signal through different Neuropilins (Nrp1 and Nrp2 respectively) and L1cam only appears important for Sema3a signaling, while Nrcam forms a complex with Nrp2. Further complexity arises from crosstalk of other families of ligands (e.g., VEGF) with Sema3 receptor components. Thus the Sema3s, which have been shown as antagonists for each other, can also act as antagonists for other families of molecules. This review compiles experimental evidence describing the receptor components for the Sema3s, detailing the current state of knowledge of which components are important for signaling of each Sema3 before going on to consider possible future directions for the field.

Presence of histone H3 acetylated at lysine 9 in male germ cells and its distribution pattern in the genome of human spermatozoa

During spermatogenesis, approximately 85% of histones are replaced by protamines. The remaining histones have been proposed to carry essential marks for the establishment of epigenetic information in the offspring. The aim of the present study was to analyse the expression pattern of histone H3 acetylated at lysine 9 (H3K9ac) during normal and impaired spermatogenesis and the binding pattern of H3K9ac to selected genes within ejaculates. Testicular biopsies, as well as semen samples, were used for immunohistochemistry. Chromatin immunoprecipitation was performed with ejaculated sperm chromatin. HeLa cells and prostate tissue served as controls. Binding of selected genes was evaluated by semiquantitative and real-time polymerase chain reaction. Immunohistochemistry of H3K9ac demonstrated positive signals in spermatogonia, spermatocytes, elongating spermatids and ejaculated spermatozoa of fertile and infertile men. H3K9ac was associated with gene promoters (CRAT, G6PD, MCF2L), exons (SOX2, GAPDH, STK11IP, FLNA, PLXNA3, SH3GLB2, CTSD) and intergenic regions (TH) in fertile men and revealed shifts of the distribution pattern in ejaculated spermatozoa of infertile men. In conclusion, H3K9ac is present in male germ cells and may play a role during the development of human spermatozoa. In addition, H3K9ac is associated with specific regions of the sperm genome defining an epigenetic code that may influence gene expression directly after fertilisation.

PlexinA4 distribution in the adult rat spinal cord and dorsal root ganglia

PlexinsA1-A4 participate in class 3 semaphorin signaling as co-receptors to neuropilin 1 and 2, PlexinA4 being the latest member of the PlexinA subfamily to be identified. Little is known about the cellular distribution of PlexinA4 in the spinal cord and dorsal root ganglion (DRG). Here, immunohistochemical studies using antibodies to PlexinA4 revealed immunolabeling in neurons in both dorsal and, to a greater extent, ventral horns of the spinal cord. Ventral horn PlexinA4 positive neurons exhibited morphology, size, and location consistent with both motor neurons and interneurons. Labeling was found in motor axons exiting through the ventral roots, and more widespread labeling was observed in ascending and descending white matter tracts. Within the DRG, immunostaining was observed in neuronal cell bodies as well as the central and peripheral processes of these cells. PlexinA4 is expressed in the peripheral nervous system where its expression is regulated upon nerve injury. This is the first detailed description of the cellular and subcellular distribution of PlexinA4 in the adult spinal cord and DRG, and it will set the basis for future studies on the potential role of PlexinA4 in regeneration and repair of the adult central and peripheral nervous system.

Estrogen up-regulation of semaphorin 3F correlates with sympathetic denervation of the rat uterus

Current evidence indicates that rises in systemic levels of estrogen create in the uterus an inhibitory environment for sympathetic nerves. However, molecular insights of these changes are far from complete. We evaluated if semaphorin 3F mRNA, a sympathetic nerve repellent, was produced by the rat uterus and if its expression was modulated by estrogen. We also analyzed whether uterine nerves express the semaphorin 3F binding receptor, neuropilin-2. Uterine levels of semaphorin 3F mRNA were measured using real time reverse transcriptase-polymerase chain reaction in prepubertal rat controls and following chronic estrogen treatment. Localization of semaphorin 3F transcripts was determined by in situ hybridization and the expression of neuropilin-2 was assessed by immunohistochemistry. These studies showed that: (1) chronic estrogen treatment led to a 5-fold induction of semaphorin 3F mRNA in the immature uterus; (2) estrogen provoked a tissue-specific induction of semaphorin 3F which was particularly localized in the connective tissue that borders muscle bundles and surrounds intrauterine blood vessels; (3) two major cell-types were recognized in the areas where transcripts were concentrated, fibroblast-like cells and infiltrating eosinophil leukocytes; and (4) some delicate nerve terminal profiles present in the estrogenized uterus were immunoreactive for neuropilin-2. Temporal and spatial expression patterns of semaphorin 3F/neuropilin-2 are consistent with a possible role of this guidance cue in the remodeling of uterine sympathetic innervation by estrogen. Though correlative in its nature, these data support a model whereby semaphorin 3F, in combination with other inhibitory molecules, converts the estrogenized myometrium to an inhospitable environment for sympathetic nerves.

Frizzled3 is required for neurogenesis and target innervation during sympathetic nervous system development

The sympathetic nervous system has served as an amenable model system to investigate molecular mechanisms underlying developmental processes in the nervous system. While much attention has been focused on neurotrophic factors controlling survival and connectivity of postmitotic sympathetic neurons, relatively little is known about signaling mechanisms regulating development of sympathetic neuroblasts. Here, we report that Frizzled3 (Fz3), a member of the Wnt receptor family, is essential for maintenance of dividing sympathetic neuroblasts. In Fz3(-/-) mice, sympathetic neuroblasts exhibit decreased proliferation and premature cell cycle exit. Fz3(-/-) sympathetic neuroblasts also undergo enhanced apoptosis, which could not be rescued by eliminating the proapoptotic factor, Bax. These deficits result in reduced generation of sympathetic neurons and pronounced decreases in the size of sympathetic chain ganglia. Furthermore, the axons of sympathetic neurons that persist in Fz3(-/-) ganglia are able to extend out of sympathetic ganglia toward distal targets, but fail to fully innervate final peripheral targets. The cell cycle exit, but not target innervation, defects in Fz3(-/-) mice are phenocopied in mice with conditional ablation of β-catenin, a component of canonical Wnt signaling, in sympathetic precursors. Sympathetic ganglia and innervation of target tissues appeared normal in mice lacking a core planar cell polarity (PCP) component, Vangl2. Together, our results suggest distinct roles for Fz3 during sympathetic neuron development; Fz3 acts at early developmental stages to maintain a pool of dividing sympathetic precursors, likely via activation of β-catenin, and Fz3 functions at later stages to promote innervation of final peripheral targets by postmitotic sympathetic neurons.

p75NTR is an obligate signaling receptor required for cues that cause sympathetic neuron growth cone collapse

The p75 neurotrophin receptor (p75NTR) is required for the activity of growth cone collapsing factors such as Nogo, MAG, OMgP, and ephrin A. Specifically, p75NTR associates with the Nogo receptor and GPI-linked ephrin A, and unliganded p75NTR mediates the biological effects of those proteins. Here we assess the requirement for p75NTR for the growth cone collapsing responses of semaphorins (Sema) 3A and 3F and ephrin B2 in sympathetic neurons. We show that the ability of Sema 3s or ephrin B2 to collapse growth cones is suppressed in p75NTR-/- sympathetic neurons. Ectopic expression of p75NTR restores the collapsing activity of Sema 3 in p75NTR-/- neurons. Moreover, p75NTR must be bound to its neurotrophin ligands to participate in Sema 3-mediated collapse. Ligand-bound p75NTR participates in Sema 3 and ephrin B2-mediated collapse via the Rho signaling pathway, since inhibition of Rho signaling is sufficient to suppress the effects of Sema 3s and ephrin B2 in p75NTR+/+ but not p75NTR-/- neurons. Our data suggest that in addition to its role as a co-receptor, p75NTR may provide an obligate parallel neurotrophin-activated inhibitory pathway that broadly sensitizes neurons to inhibitory cues.

Immunohistochemical Distribution of PlexinA4 in the Adult Rat Central Nervous System

PlexinA4 is the latest member to be identified of the PlexinA subfamily, critical transducers of class 3 semaphorin signaling as co-receptors to neuropilins 1 and 2. Despite functional information regarding the role of PlexinA4 in development and guidance of specific neuronal pathways, little is known about its distribution in the adult central nervous system (CNS). Here we report an in depth immunohistochemical analysis of PlexinA4 expression in the adult rat CNS. PlexinA4 staining was present in neurons and fibers throughout the brain and spinal cord, including neocortex, hippocampus, lateral hypothalamus, red nucleus, facial nucleus, and the mesencephalic trigeminal nucleus. PlexinA4 antibodies labeled fibers in the lateral septum, nucleus accumbens, several thalamic nuclei, substantia nigra pars reticulata, zona incerta, pontine reticular region, as well as in several cranial nerve nuclei. This constitutes the first detailed description of the topographic distribution of PlexinA4 in the adult CNS and will set the basis for future studies on the functional implications of PlexinA4 in adult brain physiology.

Neuropilin, you gotta let me know: should I stay or should I go?

Neuropilins are highly conserved single pass transmembrane proteins specific to vertebrates. They were originally identified as adhesion molecules in the nervous system, but were subsequently rediscovered as the ligand binding subunit of the class 3 semaphorin receptor in neurons and then as blood vessel receptors for the vascular endothelial growth factor VEGF. More recently they have also been implicated as mediators of the T-cell immune response and as key prognostic markers in several types of cancer. Because neuropilins bind multiple ligands and associate with several different types of co-receptors, they variably promote cell adhesion, repulsion or attraction. Which response they ultimately invoke is decided by the cellular and even subcellular context the neuropilins find themselves in. Here, we review how the developmental functions of the neuropilins are influenced by such different contexts.

Neuropilin receptors guide distinct phases of sensory and motor neuronal segmentation

The segmented trunk peripheral nervous system is generated by ventrally migrating neural crest cells that exclusively invade the anterior sclerotome and differentiate into metameric dorsal root and sympathetic ganglia. Meanwhile, ventral spinal motor axons also project through the somites in a segmental fashion. How peripheral nervous system segmentation is generated is unknown. We previously showed that neuropilin 2 (Nrp2)/semaphorin 3F (Sema3F) signaling is required for segmental neural crest migration, but not for metameric dorsal root gangliogenesis. We now expand these results to show that Nrp2 patterns initial motor axon outgrowth as well. Later, Nrp1/Sema3A signaling is essential for segmental dorsal root gangliogenesis and motor axonal fasciculation into ventral roots. Strikingly, Nrp/Sema signaling is not required for sympathetic ganglia segmentation. These data show that Nrp2 and Nrp1 work together to produce segmentation of sensory and motor nerves, and that dorsal peripheral nervous system metamerism is generated in a stepwise, Nrp-dependent process.

Neuropilin-mediated neural crest cell guidance is essential to organise sensory neurons into segmented dorsal root ganglia

The peripheral nervous system (PNS) of higher vertebrates is segmented to align the spinal nerve roots with the vertebrae. This co-patterning is set up during embryogenesis, when vertebrae develop from the sclerotome layer of the metameric somites, and PNS neurons and glia differentiate from neural crest cells (NCCs) that preferentially migrate into the anterior sclerotome halves. Previous analyses of mice deficient in the class 3 semaphorin (SEMA3) receptors neuropilin (NRP) 1 or 2 raised the possibility that each controlled a distinct aspect of trunk NCC migration. We now demonstrate that both pathways act sequentially in distinct NCC subpopulations and thereby cooperate to enforce segmental NCC migration. Specifically, SEMA3A/NRP1 signalling first directs one population of NCCs from the intersomitic path into the sclerotome, and SEMA3F/NRP2 signalling acts subsequently to restrict a second population to the anterior half of the sclerotome. NCC exclusion from either the posterior sclerotome or the intersomitic boundary is sufficient to enforce the separation of neighbouring NCC streams and the segregation of sensory NCC progeny into metameric dorsal root ganglia (DRG). By contrast, the combined loss of both guidance pathways leads to ectopic invasion of the intersomitic furrows and posterior sclerotome halves, disrupting metameric NCC streaming and DRG segmentation.

Neuropilin 1 signaling guides neural crest cells to coordinate pathway choice with cell specification

Neural crest cells (NCCs) are highly motile embryonic stem cells that delaminate from the neuroectoderm early during vertebrate embryogenesis and differentiate at defined target sites into various essential cell types. To reach their targets, NCCs follow 1 of 3 sequential pathways that correlate with NCC fate. The firstborn NCCs travel ventrally alongside intersomitic blood vessels to form sympathetic neuronal progenitors near the dorsal aorta, while the lastborn NCCs migrate superficially beneath the epidermis to give rise to melanocytes. Yet, most NCCs enter the somites to form the intermediate wave that gives rise to sympathetic and sensory neurons. Here we show that the repulsive guidance cue SEMA3A and its receptor neuropilin 1 (NRP1) are essential to direct the intermediate wave NCC precursors of peripheral neurons from a default pathway alongside intersomitic blood vessels into the anterior sclerotome. Thus, loss of function for either gene caused excessive intersomitic NCC migration, and this led to ectopic neuronal differentiation along both the anteroposterior and dorsoventral axes of the trunk. The choice of migratory pathway did not affect the specification of NCCs, as they retained their commitment to differentiate into sympathetic or sensory neurons, even when they migrated on an ectopic dorsolateral path that is normally taken by melanocyte precursors. We conclude that NRP1 signaling coordinates pathway choice with NCC fate and therefore confines neuronal differentiation to appropriate locations.

Plexin A3 and plexin A4 convey semaphorin signals during facial nerve development

In vertebrates, class 3 semaphorins (SEMA3) control axon behaviour by binding to neuronal cell surface receptors composed of a ligand binding subunit termed neuropilin (NRP) and a signal transduction subunit of the A-type plexin family (PLXNA). We have determined the requirement for SEMA3/NRP/PLXN signalling in the development of the facial nerve, which contains axons from two motor neuron populations, branchiomotor and visceromotor neurons. Loss of either SEMA3A/NRP1 or SEMA3F/NRP2 caused defasciculation and ectopic projection of facial branchiomotor axons. In contrast, facial visceromotor axons selectively required SEMA3A/NRP1. Thus, the greater superficial petrosal nerve was defasciculated, formed ectopic projections and failed to branch in its target area when either SEMA3A or NRP1 were lost. To examine which A-type plexin conveyed SEMA3/neuropilin signals during facial nerve development, we combined an expression analysis with loss of function studies. Even though all four A-type plexins were expressed in embryonic motor neurons, PLXNA1 and PLXNA2 were not essential for facial nerve development. In contrast, loss of PLXNA4 phenocopied the defects of SEMA3A and NRP1 mutants, and loss of PLXNA3 phenocopied the defects of SEMA3F and NRP2 mutants. The combined loss of PLXNA3 and PLXNA4 impaired facial branchiomotor axon guidance more severely than loss of either plexin alone, suggesting that SEMA3A and SEMA3F signals, even though both essential, are partially redundant.

Dorsal turning of motor corticospinal axons at the pyramidal decussation requires plexin signaling

BACKGROUND

The development of the corticospinal tract (CST) in higher vertebrates relies on a series of axon guidance decisions along its long projection pathway. Several guidance molecules are known to be involved at various decision points to regulate the projection of CST axons. However, previous analyses of the CST guidance defects in mutant mice lacking these molecules have suggested that there are other molecules involved in CST axon guidance that are yet to be identified. In this study, we investigate the role of plexin signaling in the guidance of motor CST axons in vivo.

RESULTS

Expression pattern studies show that plexin-A3, plexin-A4, and neuropilin-1 are expressed in the developing cerebral cortex when the motor CST axons originating from layer V cortical neurons are guided down to the spinal cord. By analyzing mutant mice, we show that motor CST axons that turn dorsally to cross the midline at the pyramidal decussation require plexin-A3 and plexin-A4 signaling. Although other CST guidance defects are found in neuropilin-1 mutants, this dorsal turning defect is not observed in either neuropilin-1 or neuropilin-2 mutants, suggesting that the local cues that activate plexin signaling at the dorsal turning point are membrane-bound semaphorins. Further expression pattern study and mutant analysis indicate that Sema6A is one of the local cues for motor CST axon turning at the pyramidal decussation.

CONCLUSION

Dorsal turning and midline crossing at the pyramidal decussation is a crucial step to properly direct CST axons into the dorsal spinal cord. We show that the signaling of plexin-A3, plexin-A4, and Sema6A is at least partially required for dorsal turning of the CST axons, while neuropilin-1 is required for proper fasciculation of the tract at midline crossing. Together with previous reports, these results demonstrate that several guidance cues are specifically utilized to regulate the dorsal turning and midline crossing of developing CST axons.

Plexin signaling selectively regulates the stereotyped pruning of corticospinal axons from visual cortex

Neurons in the developing CNS tend to send out long axon collaterals to multiple target areas. For these neurons to attain specific connections, some of their axon collaterals are subsequently pruned-a process called stereotyped axon pruning. One of the most striking examples of stereotyped pruning in the CNS is the pruning of corticospinal tract (CST) axons. The long CST collaterals from layer V neurons of the visual and motor cortices are differentially pruned during development. Here we demonstrate that select plexins and neuropilins, which serve as coreceptors for semaphorins, are expressed in visual cortical neurons at the time when CST axon collaterals are stereotypically pruned. By analyzing mutant mice, we find that the pruning of visual, but not motor, CST axon collaterals depends on plexin-A3, plexin-A4, and neuropilin-2. Expression pattern study suggests that Sema3F is a candidate local cue for the pruning of visual CST axons. Using electron microscopic analysis, we also show that visual CST axon collaterals form synaptic contacts in the spinal cord before pruning and that the unpruned collaterals in adult mutant mice are unmyelinated and maintain their synaptic contacts. Our results indicate that the stereotyped pruning of the visual and motor CST axon collaterals is differentially regulated and that this specificity arises from the differential expression of plexin receptors in the cortex.